Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. The extraction of bitumen from sand using hot water processes yields large volumes of fine tailings composed of fine silts, clays, residual bitumen and water. Mineral fractions with a particle diameter less than 44 microns are referred to as “fines.” These fines are typically clay mineral suspensions, predominantly kaolinite and illite.
The fine tailings suspension is typically 85% water and 15% fine particles by mass. Dewatering of fine tailings occurs very slowly. When first discharged in ponds, the very low density material is referred to as thin fine tailings. After a few years when the fine tailings have reached a solids content of about 30-35%, they are referred to as fluid fine tailings which behave as a fluid-like colloidal material. The fact that fluid fine tailings behave as a fluid and have very slow consolidation rates significantly limits options to reclaim tailings ponds. A challenge facing the industry remains strengthening the oil sand tailings deposits so that they can be reclaimed and no longer require containment.
Recently, the present applicant developed a process for dewatering oil sands tailings by treating the tailings with coagulant and flocculant prior to dewatering by centrifugation (see U.S. patent application Ser. No. 13/594,402, incorporated hereto by reference). The centrifugation process is particularly useful with, but not limited to, fluid fine tailings. Dewatering the tailings by centrifugation enables reclamation of tailings disposal areas and recovery of water for recycling. However, one challenge faced by the applicant is disposal of the resultant centrifuge cake. Cake properties are a function of the solids content and water chemistry. It was discovered that, while the addition of gypsum improved the conveyability of the cake from the centrifuge, typically, the produced cake from the centrifugation process still only had a solids content of about 50 to 55% by weight (about 31% by volume) and a shear strength of about 0.5 to 2.0 kPa. Thus, further strengthening of the centrifuge cake is desirable.
Accordingly, there is a need for an improved method to treat fine tailings to strengthen same and reduce their water content in order to reclaim the land on which fine tailings are disposed.